Field of the Invention
The present invention relates to a mode converter, and particularly relates to a technique that is used in a waveguide for a communication using a millimeter-wave band.
Description of the Related Art
In recent years, high-capacity and high-speed communication of several Gbps using a millimeter-wave band has been proposed, and some are being currently implemented. In particular, wireless communication devices operating in the 60 GHz band have become increasingly more important. Such devices are expected to become widespread in the consumer field since a broad frequency band up to 57-66 GHz can be used without a license, and therefore, realizing an inexpensive and compact millimeter-wave communication module is in urgent need.
As forms of implementing a compact and low-cost millimeter-wave communication module, in R. Suga, et al. “Cost-Effective 60-GHz Antenna-Package with End-Fire Radiation from Open-Ended Post-Wall Waveguide for Wireless File-Transfer System”, 2010 IEEE MTT-S International Microwave Symposium, pp. 449-452 and Japanese Unexamined Patent Application, First Publication No. 2011-109438, a millimeter-wave module using a waveguide (PWA Post-wall Waveguide Antenna) by a printed wiring board is disclosed.
As shown in Japanese Unexamined Patent Application, First Publication No. 2011-109438, in the technique described above, the sidewalls (metal walls) of the conventional waveguide are replaced with a group of through-holes (post group) of the printed wiring board. A wireless communication IC (CMOS-IC) is mounted on the PWA. The millimeter-wave signals which are outputted from the wireless communication IC (referred to as a semiconductor chip in the specification of Japanese Unexamined Patent Application, First Publication No. 2011-109438 and the same applies hereinafter) by methods of wire bonding, a bump connection and the like are once transmitted through transmission lines (described as a line of a microstrip, a coplanar, strip and the like) of a plane circuit. Ultimately, the signals are guided to a waveguide structure portion (described as a waveguide) through a plane circuit and waveguide conversion structure (described as a center conductor).
FIG. 35 is a cross-sectional view showing an exemplary configuration of a conventional mode converter (converter). As shown in FIG. 35, in the converter 810, the waveguide 802 has an opening 825 that emits radio waves at a front end surface (right-hand side in FIG. 35). The waveguide 802 is configured with a plurality of post (column) walls 820 and ground conductor layers (copper foils) 821, 822. In the waveguide 802, a pin (plane circuit or waveguide converter) 823 is inserted as a power source. Millimeter-wave signals that are introduced into the pin 823 from the transmission line 824 are emitted as electromagnetic waves from the opening portion 825 which is formed ahead of the waveguide 802. The converter 810 which is an antenna package is stacked and formed in a multiple-layer manner via conductors 827A, 827B and a plurality of substrates 828A, 828B, 828C which are dielectric. The pin 823 is formed such that vias are formed in advance in the substrates 828B and 828C, and then the substrates are laminated.
In a high frequency circuit in general, impedance matching is necessary when two circuits are connected. This means that signals can be transmitted without being reflected at a connection point of the two circuits. In particular, at a connection point of a plane circuit or a transmission line as a first circuit and a waveguide as a second circuit, it is necessary to transmit the signals without reflection. In the structure shown in FIG. 35, in a predetermined frequency band, the impedance is matched by adjusting the length of the pin 823 to a predetermined value to realize signal transmission where the reflection loss is reduced. In addition, as one of the impedance matching methods, there is a method of optimizing a distance between the pin 823 and the ground conductor layer 822.
In conventional methods of manufacturing a converter, since vias are formed in advance in a plurality of substrates each having a fixed thickness and a pin is manufactured by stacking the plurality of substrates, the length of the pin can have only discrete values depending on the thickness of the substrates and the impedance adjustment is not easy. In addition, the thickness of each substrate to be stacked cannot be determined individually and depends on such as the availability of materials thereof. Therefore, it is not easy to realize a pin having an optimal length.
In addition, the pin in the conventional converter is configured in the substrate. Therefore, in practice, it is difficult to check to which position the pin is extended in the substrate. In particular, after the converter is completed, a distance between the pin 823 and the ground conductor layer 822 cannot be adjusted (see Japanese Unexamined Patent Application, First Publication No. 2011-082337).
In the structure described above, due to the influence of an adhesive used when the substrates are stacked and a positional deviation of the conductor during stacking the substrates, reflection characteristics are deteriorated and the like and the characteristics as designed are not be obtained, thereby causing an increase of a loss in the waveguide and the like.
Furthermore, the conventional converters include a structure where a plurality of substrates in which a via is formed are stacked. Therefore, unfavorable conditions arise such as an increase of the number of processing steps, an increase of transmission loss due to an adhesive, variations of transmission characteristics caused by materials of the respective layers, and a difficulty in obtaining the materials.
The present invention is made in consideration of the above-described points, and provides a mode converter which includes an optimum pin and is capable of being easily checked and adjusting a distance between a ground conductor layer and a tip of a pin. In addition, the present invention provides a mode converter having a high manufacturing efficiency by reducing the number of processing steps to have optimal characteristics.